Exchange rates determine how much one currency is worth relative to another, and they affect everything from the price of imports to the returns on foreign investments. Understanding the theories and factors behind exchange rate movements is central to international finance.
This section covers the major theories of exchange rate determination, the economic and political factors that drive currency values, and the practical implications of purchasing power parity and interest rate parity.
Exchange Rate Determination Theories
Main Theories
Purchasing power parity (PPP) suggests that exchange rates adjust so that identical goods cost the same across countries when priced in a common currency. If prices rise faster in one country than another, that country's currency should depreciate to restore balance.
Interest rate parity (IRP) holds that the difference in interest rates between two countries should be offset by changes in the exchange rate. If Country A offers higher interest rates than Country B, Country A's currency is expected to depreciate by roughly the amount of that interest rate gap. Otherwise, investors could earn risk-free profits just by moving money between countries.
Balance of payments (BOP) theory frames exchange rates as the result of supply and demand for currencies. When a country runs a trade surplus, foreign buyers need its currency to pay for exports, pushing the currency's value up. Capital flows work the same way: foreign investment flowing into a country increases demand for its currency.
Deviations from Theories
These theories describe long-run tendencies, not precise predictions. In practice:
- PPP deviations arise from transportation costs, tariffs, differences in product quality, and the fact that many goods and services simply aren't traded across borders.
- IRP deviations occur because of transaction costs, capital controls, political risk, and differences in the perceived creditworthiness of countries. These frictions prevent the instant arbitrage that the theory assumes.
Economic Factors and Exchange Rates
Macroeconomic Indicators
- Inflation rates: Countries with higher inflation tend to see their currencies weaken. If U.S. inflation runs at 2% while Turkey's runs at 50%, the Turkish lira will lose purchasing power relative to the dollar over time.
- Interest rates: Higher interest rates attract foreign capital, increasing demand for the domestic currency and pushing its value up. This is why currency markets react sharply to central bank rate decisions.
- Economic growth: Strong GDP growth tends to attract foreign investment, which increases demand for the domestic currency. Rapidly growing emerging markets often experience currency appreciation for this reason, though the relationship isn't automatic.
Political and Fiscal Factors
- Political stability: Investors move capital away from countries experiencing political turmoil, weakening those currencies. Venezuela and Zimbabwe are extreme examples where political crises contributed to dramatic currency collapses.
- Government debt: High debt levels raise concerns about a government's ability to meet its obligations, which can drive investors away and weaken the currency. Greece during the eurozone debt crisis and Argentina's repeated debt defaults illustrate this dynamic.
- Monetary policy: Central bank actions directly affect exchange rates. When the U.S. Federal Reserve raises interest rates, the dollar typically strengthens. When the European Central Bank engages in quantitative easing (buying bonds to inject money into the economy), the euro tends to weaken.
Purchasing Power Parity
Concept and Theories
Purchasing power parity is built on a simple idea: the exchange rate between two currencies should reflect the ratio of price levels in the two countries.
- Absolute PPP says the exchange rate should equalize the price of a comparable basket of goods across countries. If that basket costs $100 in the U.S. and £80 in the U.K., the implied exchange rate is $1.25/£.
- Relative PPP is more practical. It doesn't require prices to be equal across countries; instead, it says the rate of change in the exchange rate should reflect the difference in inflation rates. If U.S. inflation is 3% and eurozone inflation is 1%, relative PPP predicts the dollar will depreciate by roughly 2% against the euro.
Law of One Price
The law of one price is the foundation of PPP. It states that an identical good should sell for the same price in different markets once you convert to a common currency.
For example, if a widget costs $10 in the U.S. and €8 in Europe, the law of one price implies an exchange rate of . If the actual exchange rate differs, there's a theoretical profit opportunity from buying the widget where it's cheap and selling where it's expensive.
Applications and Limitations
PPP is widely used to compare living standards across countries. Organizations like the World Bank use PPP-adjusted GDP figures because raw exchange rates can be misleading. PPP also helps analysts judge whether a currency is overvalued or undervalued relative to its "fair" level.
That said, PPP has real limitations:
- Consumption patterns differ across countries (people in different places buy different things).
- Trade barriers and transportation costs prevent goods from flowing freely.
- Many services are non-tradable. You can't ship a haircut or an apartment from a cheap country to an expensive one.
These frictions mean PPP works better as a long-run anchor than a short-term predictor.
Interest Rate Parity and Implications
Interest Rate Parity Theories
Interest rate parity says the interest rate differential between two countries should equal the expected change in the exchange rate between their currencies. There are two versions:
- Covered interest rate parity (CIRP): The forward exchange rate should incorporate the interest rate differential. If U.S. rates are 5% and Japanese rates are 1%, the forward rate on yen should price in a 4% appreciation of the yen against the dollar. CIRP uses forward contracts, so exchange rate risk is hedged.
- Uncovered interest rate parity (UIRP): The expected future spot rate should adjust by the interest rate differential. Unlike CIRP, there's no hedging here, so UIRP involves taking on exchange rate risk. Empirically, UIRP holds less reliably than CIRP.
Arbitrage and Market Efficiency
IRP implies that you can't earn risk-free profits through a simple sequence of borrowing in one currency, converting, investing at a higher rate abroad, and converting back. If IRP holds, the exchange rate movement (or forward rate) exactly offsets the interest rate advantage.
When IRP holds, the foreign exchange market is considered efficient. Any temporary mispricings get corrected quickly as traders exploit them, pushing rates back into alignment.
Implications for International Finance
IRP connects interest rates, inflation expectations, and exchange rates into a coherent framework. This matters for several reasons:
- Carry trades emerge when IRP doesn't hold perfectly. In a carry trade, investors borrow in a low-interest-rate currency (the Japanese yen has been a classic funding currency) and invest in a high-interest-rate currency, profiting as long as the high-rate currency doesn't depreciate enough to wipe out the interest rate advantage. Carry trades can be profitable for extended periods but are vulnerable to sudden reversals.
- Policy decisions by central banks take interest rate differentials into account. Raising domestic rates can attract foreign capital and strengthen the currency, but it also affects borrowing costs at home. Managing this tradeoff is a core challenge for policymakers.
- Corporate hedging relies on IRP relationships. Multinational firms use forward contracts priced according to CIRP to lock in exchange rates for future transactions, reducing uncertainty in cross-border business.